Source

File: wp-includes/class-wp-xmlrpc-server.php

protected function _prepare_user( $user, $fields ) {
		$_user = array( 'user_id' => strval( $user->ID ) );

		$user_fields = array(
			'username'     => $user->user_login,
			'first_name'   => $user->user_firstname,
			'last_name'    => $user->user_lastname,
			'registered'   => $this->_convert_date( $user->user_registered ),
			'bio'          => $user->user_description,
			'email'        => $user->user_email,
			'nickname'     => $user->nickname,
			'nicename'     => $user->user_nicename,
			'url'          => $user->user_url,
			'display_name' => $user->display_name,
			'roles'        => $user->roles,
		);

		if ( in_array( 'all', $fields, true ) ) {
			$_user = array_merge( $_user, $user_fields );
		} else {
			if ( in_array( 'basic', $fields, true ) ) {
				$basic_fields = array( 'username', 'email', 'registered', 'display_name', 'nicename' );
				$fields       = array_merge( $fields, $basic_fields );
			}
			$requested_fields = array_intersect_key( $user_fields, array_flip( $fields ) );
			$_user            = array_merge( $_user, $requested_fields );
		}

		/**
		 * Filters XML-RPC-prepared data for the given user.
		 *
		 * @since 3.5.0
		 *
		 * @param array   $_user  An array of user data.
		 * @param WP_User $user   User object.
		 * @param array   $fields An array of user fields.
		 */
		return apply_filters( 'xmlrpc_prepare_user', $_user, $user, $fields );
	}